With respect to the efforts which are being made to provide airplanes which conform to future ecological requirements and are inexpensive to produce and operate, and to nevertheless meet the strictest safety requirements, possible ways are increasingly being sought to produce the essential primary structures (e.g. wings, fuselage components, housing for the drive units, etc.) using fiber-reinforced composite material rather than aluminum. This lightweight construction technique makes it possible, in particular, to considerably reduce the weight of the airplanes. During the production of such essential primary structures, it must be taken into account that these take on a considerable scale; by way of example, the landing flaps are structural parts which extend over a number of meters. These structural parts are additionally exposed to high levels of stress during operation and therefore represent safety-critical structural parts, for which special quality requirements have to be observed.
Fiber-reinforced composite materials of this type generally comprise two essential components, namely firstly the fibers and secondly a polymer matrix which surrounds the fibers. The matrix encompasses the fibers and is cured by a thermal treatment (polymerization), such that three-dimensional cross-linking takes place. This polymerization has the effect that the fibers are bonded firmly to one another and therefore forces can be introduced into the fibers, namely predominantly via shear stresses. Suitable fibers are both carbon fibers and possibly also glass fibers. Carbon fibers, which nowadays are still relatively expensive, regularly comprise carbon to an extent of at least 90% by weight. The diameter of the fibers is, for example, 4.5 to 8 μm (micrometer). Carbon fibers of this type have anisotropic properties. By contrast, glass fibers have an amorphous structure and isotropic properties. They predominantly consist of silicon oxide, it being possible for further oxides to be admixed if appropriate. Whereas the glass fibers are relatively inexpensive, the carbon fibers are noted for their high strength and rigidity.
Particularly in the construction of airplanes, what is known as pre-preg technology is employed. In this technology, for example, pre-impregnated fabrics or other fiber forms (preform) are soaked in synthetic resins and thermally treated merely until they solidify slightly (gel formation), such that they can be handled in layers. A pre-preg material of this type exhibits a small degree of adhesion and can therefore be arranged readily in appropriate molding tools or one on top of another in layers, until the desired form of the structural part is formed. When the desired layers of the pre-preg material are arranged, they can be (thermally) cured. In order to cure said pre-preg structural parts, use is presently made of what are known as autoclaves, i.e. ovens which may have to be heated with an overpressure (up to 10 bar) over many hours in order to achieve complete curing of the structural parts.
In addition, DE 10 2005 050 528 A1, the contents of which are incorporated by reference, discloses a microwave autoclave, with which the production of fiber composite structural parts by microwave radiation is proposed. The apparatus proposed in said document makes it possible to couple microwave radiation into the pressure chamber of the autoclave. The direct excitation of the pre-preg materials suitable for this method with microwaves has the advantage that it is not necessary to heat the air located in the autoclave or the inert gas located therein, which is present in a considerable volume owing to the size of the structural parts. The use of microwave technology makes it possible to heat the pre-preg material to be cured itself directly, and the rest of the surrounding region accordingly remains relatively cold.
The microwave resonator described in DE 103 29 411 A1, the contents of which are incorporated by reference, is likewise suitable for carrying out the thermal treatment. Said microwave resonator is generally operated without an overpressure. However, it may also be integrated in a pressure vessel (autoclave).
When heating a material using microwaves, the following active mechanisms may set in: dielectric heating and resistive heating. If (freely) movable dipoles (i.e. molecules having an irregular charge distribution) are present in the material, these are excited to oscillate at a high frequency in an electromagnetic field produced by the microwaves. This kinetic energy of the dipoles is then converted by internal friction into heat, which is produced directly in the material (dielectric heating). In addition, it is also possible for eddy currents to arise as a result of induction, and therefore the electrical resistance of the material finally causes an increase in temperature (resistive heating). By way of example, the material can thus be heated to temperatures above 130° C., above 160° C. or even also above 200° C. This temperature level makes it possible for the polymerization or curing of the pre-preg materials to set in.
In this respect, airplane components having a relatively large base surface and elevations protruding therefrom are in the foreground in particular. By way of example, said elevations are web-like fins intended to contribute, in particular, to an increase in the rigidity of the (assembled) component. Merely by way of example, elevations of this type may have a length of about 11 m (meter), a material thickness in the range of approximately 2.5 mm to 4 mm (millimeter) and a height, with which they protrude beyond the base surface, of at least 25 mm (millimeter).
During the production of such components by curing by microwave irradiation, it is important that uniform and sufficient crosslinking is ensured in materials. For this purpose, it is essential that this “contorted” shape of the component can be treated appropriately with microradiation and/or that a homogeneous temperature distribution is achieved on the component. It must also be taken into consideration that the pre-preg materials used here are regularly themselves not dimensionally stable, i.e. have to be fixed in the desired position using appropriate holding and/or molding tools during the microwave irradiation. The problem indicated above is thereby enlarged further.